A frozen Earth-like planet circling just one of a pair of stars has greatly improved the prospects of finding extra-terrestrial life, astronomers claim.
Until the discovery, no-one realised that rocky planets could form stable orbits round one member of a binary star system. Planets orbiting pairs of binary stars are relatively common.
US astronomer Professor Scott Gaudi, from Ohio State University, said: "This greatly expands the potential locations to discover habitable planets in the future.
"Half the stars in the galaxy are in binary systems. We had no idea if Earth-like planets in Earth-like orbits could even form in these systems."
The new planet, located 3,000 light years away, is twice as massive as Earth and almost exactly the same distance from its parent star as the Earth is from the Sun.
Because the host star shines 400 times less brightly than the Sun it is a frozen and almost certainly lifeless world.
But the astronomers point out that the same planet orbiting a Sun-like star would be in the "habitable zone" where conditions are potentially suitable for liquid surface water and life.
Prof Gould led four international teams reporting on the discovery in the journal Science.
The planet was found by chance when scientists spotted an unusual signal in light from a "microlensing event".
This occurs when a star's gravity acts like a lens, bending light from a much more distant object precisely behind it.
In this case, the binary system containing the new planet disturbed light from a much more distant star 20,000 light years away in the constellation Sagittarius.
The planet caused a "dip" and distortion in the light signal which after computer analysis yielded information about its mass, distance from the star, and orientation.
Data from a network of telescopes in Chile, New Zealand, Israel and Australia, were used to confirm the findings.
"In gravitational microlensing we don't even look at the light from the star-planet system," said Prof Gould. "We just observe how its gravity affects light from a more distant, unrelated star. This gives us a new tool to search for planets in binary star systems."
The second star in the binary system is also very dim. The two are only as far apart as the distance between Saturn and the Sun.
:: Two "Goldilocks" planets thought to lie in the habitable zone of a Gliese 581, a dwarf star just 20 light years from the Sun, have turned out not to exist.
Researchers found that light signals that seemed to be from the planets were actually created by events in the star itself.
A star's habitable orbital region is known as the "Goldilocks" zone because conditions there are not too hot or too cold but "just right" to support life.
Astronomers announcing the news in the journal Science were not disheartened, however.
"This result is exciting because it explains, for the first time, all the previous and somewhat conflicting observations of the intriguing dwarf star Gliese 581," said Dr Paul Robertson, from Pennsylvania State University in the US.
Gliese 581 has three other confirmed planets, none of which lie within its habitable zone.